Facile pH-Dependent Synthesis and Characterization of Catechol Stabilized Silver Nanoparticles for Catalytic Reduction of 4-Nitrophenol

Catalysis by silver nanoparticles (Ag-NPs) in organic transformations has received growing attention due to their unique reactivity and selectivity. Herein, we investigated a versatile one-step approach for synthesizing thermally stable AgNPs using catechol (1,2-benzenediol) without additional reducing and stabilizing agents in aqueous solution. In an alkaline environment, oxidation of catechol played a dual role in the reduction of silver ions (Ag⁺) and stabilization of the AgNPs. Nanoparticles with different size and morphology were obtained under different experimental conditions. X-ray diffraction (XRD) analysis suggests the formation of crystalline AgNPs of average size 13, 38 and 47 nm and face centered cubic structure as the reaction pH varied. As demonstrated in dynamic light scattering (DLS) and scanning electron microscopy (SEM) images, AgNPs with uniform size distribution (50 nm) were synthesized at pH 11. The nanoparticles are thermally stable with a steady loss of weight up to 800 °C as confirmed by thermogravimetric analysis (TGA). Comparing to AgNPs@pH5 and AgNPs@pH8, AgNPs synthesized at pH 11 have shown significant catalytic activity in the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with 61% conversion at 20 °C. The results suggested that stable and monodisperse nanoparticles with tunable catalytic activity could be produced as the pH of the reaction was altered. Graphical Abstract: [Figure not available: see fulltext.].